Dorsal lip of blastopore of frog3/15/2024 When the embryo is composed of over 10.000 blastomeres ( R.pipiens – after 25 or 26 hours), the next stage of embryonic development begins – gastrulation, which lasts around 24 hours. The R.pipiens embryo reaches its 8-cell stage in 5.5 hours at the temperature of 18 degrees Celsius, and X.laevis does so in 4.5 hours in the same temperature. Higher temperature determines a faster division rate. For example, divisions in R.pipiens embryo occur once every hour, whereas in the X.laevis embryo – once every half hour. The rate at which cleavages occur depends on the species and on the temperature of the organism’s environment. This means that the genetic material of the embryo begins transcription. At the moment in which the creation of cleavages becomes asynchronous, a midblastula transition occurs. From the stage of having 128 cells, the embryo develops a cavity, the blastocoele, and is called a blastula. Due to the fact that macromeres contain more yolk, it is easier for the micromeres to further divide themselves.Īn embryo counting 16 to 64 blastomeres is called a morula. The third cleavage runs equatorially and closer to the animal pole, thus creating blastomeres of unequal size (micromeres in the animal region and macromeres in the vegetal region). This results in the creation of four identical blastomeres - separate cells now forming the embryo. The second cleavage also cuts through the gray crescent, although always running perpendicularly to the first one. The first cleavage runs across the animal-vegetal axis, dividing the gray crescent into two parts. The cleavage (cell division) of a frog’s embryo is complete and uneven, because most of the yolk is gathered in the vegetal region. Its establishment determines the location of the dorsal and ventral (up-down) axis, as well as of the anterior and posterior (front-back) axis and the dextro-sinistral (left-right) axis of the embryo. This results in the creation of the gray crescent. A relatively short time after fertilization, the cortical cytoplasm (located just beneath the cell membrane) rotates by 30 degrees. Two blocks - defensive mechanisms meant to prevent polyspermy - occur: the fast block and the slow block. The sperm cell enters the oocyte in the region of the animal pole. The pigment is composed of light-absorbing melanin. The animal pole of the cell contains pigment cells, whereas the vegetal pole (the yolk) contains most of the nutritive material. The oocyte in these frog species is a polarized cell - it has specified axes and poles. The embryonic development of tailless amphibians is presented below using the African clawed frog ( Xenopus laevis) and the northern leopard frog ( Rana pipiens) as examples. One of the most basic criteria of such development is independence from a water habitat.Īmphibians were the earliest animals to adapt themselves to a mixed environment containing both water and dry land. Main article: Embryogenesis Early stages of embryogenesis of tailless amphibians Įmbryogenesis in living creatures occurs in different ways depending on class and species.
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